1. Field of the Invention
The present invention relates to metal wiring fabrication processes for next generation devices, and relates in particular to a method and apparatus for producing a metal deposit inside fine cavities fabricated on a substrate.
2. Description of the Related Art
In conventional semiconductor integrated circuits, elements such as transistors, resistors and capacitors are connected by aluminum wiring. Aluminum wiring is fabricated by forming an aluminum film on a substrate by sputtering process, forming a photo-resist film on the aluminum film, printing wiring patterns on the photo-resist film, and etching the aluminum film to leave wiring lines. As integration density increases, there is a demand for finer interconnection lines, but there have been some problems caused by the properties of aluminum material itself. However, the etching method is sometimes not applicable to forming wiring patterns using other metals, and a different approach is necessary. A substitute method for such cases is, to fabricate grooves and holes for wiring purposes on the substrate, and fill the cavities with metals using such methods as chemical vapor deposition (CVD), sputtering or plating, and then the unnecessary surface layer is removed by using methods such as chemical mechanical polishing (CMP).
Plating is used widely as a metal film forming technique, and has many good features. FIG. 7 shows a basic plating apparatus. A plating chamber 1 holds plating solution 9, and an anode electrode 4 holding a substrate W is opposed by a cathode electrode 3. A stirrer 11 is provided to stir plating solution 9 during plating process. Pre-plating treatment for plating includes washing or etching of the substrate. Plating is advantageous because of its relatively low cost, and high purity material can be produced using a low temperature process which produce no adverse thermal degradation in the plated product.
However, since it is difficult to force the plating solution into fine cavities fabricated on the wafer, the current plating technology is rarely able to fill deep cavities of a high aspect ratio successfully with metals. Even if the substrate is immersed in the plating solution, the cavities are occupied by residual air, and, as illustrated in FIG. 6, complete infiltration is not possible and voids are generated in the deposited metal. It is considered that wettability of the substrate material and surface tension of the plating solution have a large influence on the infiltration behavior, and the difficulty increases with decreasing width of the cavities.